Are Zoned Heating/Cooling Systems a Good Idea or a Bad Idea?

For starters, I live in Atlanta, Ga and I am not an HVAC pro so this is a serious question that I would like answered for one of my clients. I understand the premise of a zoned system and on the surface, it sounds like a good idea. Then I considered the fact that I have always told my customers that it is a bad idea to close off vents in rooms they are not in because of duct leakage. Also, the second law of thermodynamics simply states the hot goes to cold, so now the unheated areas are doing their best to rob warm air from the heated areas. Finally, if your zoning a single system, aren't you creating on oversizing issue because the unit (that was probably oversized in the first place) is now servicing a smaller are than what it was designed for?

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Some of the benefit to 2 stage A/C units is the "oversized coils" in relation to compressor size when the unit is operating on low stage. If a 2 stager unit is 2ton/3ton you have a 2ton A/C system most of the time with 3 ton sized condenser and evaporator coils. More coil surface area = better heat transfer.

I wanted to add, that a furnace or boiler that can modulate has a far greater means of tailoring capacity to load. A 5:1 modulating range or turndown range, is excellent and your 60 MBH appliance can work very well, if not more efficiently at 12 MBH input (10.8 MBH output or more), given the larger internal surface areas.

So the "it depends" part of the answer hinges a lot on what kind of appliance you have. Single stage? Not so much. :)

Hi Jon- Yes, I figured you were forced hot air and AC, but I used the HW example as to how one can mitigate the cycling issue. Also the contrast between FHA/AC and hydronic systems.
Those examples aside, the principles still hold- if your system modulates capacity, you stand a higher chance of maintaining efficiency.

Also, on the cooling side, you already know that you cannot drop your airflow below critical levels or you will freeze-over your coil. Most AC-based zoning systems, I call them 30/70 systems because they apportion all of the airflow/capacity to those approximate portions per zone. The system still puts out 100% but is directed to greatest need. These systems frequently are appropriately under-sized (say what is a 4.5 to 5 ton system on paper, might be sized at 3.5 to 4 tons based on the most likely concurrent load and not the sum of peak loads, which will never really occur.)

To modulate refrigerant capacity, the newer inverter driven compressors are outstanding, but are not widely available outside of the so-called ductless split systems, but should be available across conventional split systems in the next year or so. Until then, multi-stage seems to be the best one can do. A Rawal APR valve has great utility in modulating refrigerant flow especially in single-stage systems. A lot of good options out there.

On the combustion side, gas modulations is your only choice and the deeper the better.

Thanks for the info Brad, it was very helpful. What actually got me to ask this question came from a recent client that spent a bunch of money adding zoning to an existing system. Oversizing is a common issue here because no one did manual J's because there were very few building codes (and even fewer enforced) during the 90's building boom. This guys actual reason for doing the zoning was due to serious comfort issues in a large bonus room over the garage. He was talked into zoning by a HVAC contractor. During my audit, I found ducts running to that area that were disconnected and the rest were not properly sealed so that contractor never even inspected the condition of the current system. The homeowner has used the zoning and weatherstripping on the door to completely shut off that room and they only use it when they have company (sad but true). This has now created other issues. Basically, the homeowner wasted a bunch of money and is now reluctant to spend more money to actually fix his problems.

You are welcome, Jon.
Man, that hurts, when a client has gone down a counter-productive path. Unfortunately, a lot of well-meaning efforts turn sour in hindsight; everyone has their expertise and the big picture is lost.

What I have found is sometimes necessary is to "recombine" the zones into one large one. This means opening doors too, not only to share the conditions, but also that return air paths are rarely effective nor predictable.

If you do have a short-cycling issue on the cooling side, check out the Rawal APR device I mentioned. They run about $500-600 plus installation and requires a pull and recharge on the refrigerant side. But they can take a 3.5 ton unit down to 1.5 tons as a "top of the head" example. This means long, low, steady operation and the goal in your area -dehumidification.

2 floors should have 2 separate systems in most cases. It just makes more sense because it reduces ductwork lengths and system complexity. If one system fails the occupants can resided in the other part of the house until they can get the broke system serviced.

Our house is a single story and I have "redneck zone control" installed. It amounts to a damper in the attic that I manually adjust for summer or winter since we like having the master bedroom cooler than the rest of the house. Simple, low cost, and effective.